Crystals in the form of thin planar wafers are used with all types of electrical equipment to provide a frequency standard. The crystal is cut along a predetermined crystalline axis to provide controlled movement for vibration in response to electrical signals applied to the opposite planar surfaces of the crystal.
In order to achieve desirable frequency characteristics, the crystals are sometimes made very thin but still must be an element that is not too fragile for human handling. It is also desirable that the size of the crystal wafer along the planar direction be as large as possible to reduce the internal resistance of the crystal and thereby increase its "Q". However, the size of the crystal has practical limits, to a large degree controlled by the size limitations on the entire unit. In any event, because of the thin planar shape of the crystal, it is readily susceptible to damage, particularly when subject to shock in a direction transverse the planar surface.
With integrated circuits and micro-minature circuits, the crystal unit often is the largest component in an electrical system. Hence, there has been a continual effort to reduce the size of the entire crystal unit. The crystal unit generally includes the planar crystal mounted to a base by a pair of electrodes and a cover secured to the base for encapsulating the crystal. The container is comprised of the cover and base that have a generally oval shape and that extend along the planar direction of the crystal to allow for the planar shape of the crystal. Hence, for miniaturization purposes, the size of the container has been reduced to provide less and less space between the crystal and the cover. It was found that if the crystal structure is subjected to decelerating or accelerating forces due to shock, such as being dropped, etc., the crystal tends to bang against the cover sides often resulting in permanent damage in the form of cracks.
Crystal units are often used in radio circuits, such as for example in portable two-way radios, because they provide extremely accurate frequency control. Such two-way radios are often used by the military and police forces where they are essential for proper communications. For convenience purposes, it is desirable to make the two-way radios as small as possible, and yet at the same time as rugged as possible since such two-way radios are often subjected to extremely rough handling, even to the extent of being used as weapons or tools. In the past, the crystal was generally considered as the weakest item in the two-way radio structure and often sudden shocks destroyed the crystal and essentially rendered the two-way radio inoperative, often at the time when it was needed most. Hence, it can be seen that there is a necessity for the miniturization of crystal structures and yet a need to improve the ruggedness of the structure.
Various solutions were attempted in the prior art to minimize the effect of such shocks on the crystal. One unsuccessful method was to apply a plastic coating to the inner surface of the cover. Although such coating may have helped, the cover still functioned as a solid, non-resilient, barrier against which crystals banged, when subjected to shock, often destroying the unit. To provide a resilient cushion, a strip of plastic was placed between the container and the crystal that surrounded the crystal. However, this approach was found to be troublesome, since the strip of plastic tended to shift within the container and often abutted against the crystal interfering with its proper operation. The crystal is also often subjected to extremes in operating temperatures. The plastic strip of the prior art tended to expand and contract with temperature changes, and at times, the loose ends of the plastic strip would curl, or move, to engage the crystal and prevent its free movement and vibration.
It is very important that the crystal is continuously free to vibrate without interference when subjected to electrical signals so that the crystal can provide an accurate frequency standard. Care must also be taken to assure that the crystal is not contaminated by deposits due to time, temperature, and use. The thickness of the electrodes is placed upon the opposite planar surfaces of the crystal and are accurately controlled so that the crystal is tuned to the correct frequency of operation. These electrodes are generally deposited by vacuum deposition techniques. Generally, the final step in the deposition technique includes a frequency detection circuit which stops the deposition procedure when the crystal plus the electrodes are tuned to the correct frequency. The crystal is mounted into the container and the container is filled with an inert gas and sealed off so that no further depositions will form on the crystal and the electrode structure. If added materials or structures are to be included within the container to act, such as for example as shock absorbers, care should be taken in the selection of the material so that the material will not "outgas" depositions on the crystal over the expected range of operating temperatures. The outgassed depositions tend to change to frequency characteristics with time, particularly when subjected to higher temperatures.
It is therefore an object of this invention, to provide a new and improved shock absorber for crystal units.
It is also an object of this invention to provide a new and improved shock absorber for crystal units that does not interfere with the operation of the crystal over wide temperature ranges.
It is a further object of this invention to provide a new and improve shock absorber for crystal units that does not move within the crystal container, nor abuts against the crystal under normal operation.
It is still a further object of this invention to provide a new and improved shock absorber for crystal units that does not change in shape with variations in temperature to the extent of engaging the crystal.
It is an object of this invention to provide a new and improved shock absorber for crystal units that does not outgas over the expected operating temperature range of the crystal unit.
It is also an object of this invention to provide a new and improved shock absorber for crystal units that is inexpensive and easily assembled.